It is thought to be the first time a senior US law-enforcement official has publicly offered an opinion on whether Bin Laden, the prime suspect believed to be behind the 11 September attacks, is dead or alive.

"Is (Bin Laden) alive or is he dead?" Mr Watson said. "I am not really sure of the answer... I personally think he is probably not with us anymore but I have no evidence to support that."

The remarks, made at a law-enforcement conference in Washington on Wednesday, follow recent statements from both an Arabic newspaper editor and the chief of German foreign intelligence that Bin Laden is still alive.

Bin Laden's last video surfaced in May

But Mr Watson's comments suggest that the FBI, at least, has no firsthand information that confirms Bin Laden is still alive.

Other US officials told the Associated Press they were surprised by Mr Watson's remarks, as Washington's official position remains that it does not know where Bin Laden is, or whether he is still alive.

Abdel-Bari Atwan, editor of the London-based Al-Quds Al Arabi newspaper, said this week that the leader of the al-Qaeda leader is in good health, but had been wounded in an attack on his base in Afghanistan last December.

Mr Atwan said Bin Laden's followers had told him that he would not make more video statements until his group launches another attack on the United States.

Mr Atwan is one of few journalists to have interviewed Bin Laden before the US attacks.

Last week, the head of Germany's foreign intelligence agency also said Bin Laden was alive and believed to be hiding in the border region between Pakistan and Afghanistan.

'New attacks'

August Hanning told German newspaper Welt am Sonntag that the wanted Saudi dissident was still very much a key figure within the al-Qaeda terror network.

"Given the information we have we are convinced that Bin Laden is still alive," he said.

"He is still the figurehead of al-Qaeda, but doesn't appear to move around very much."

Mr Hanning said that an estimated 5,000 al-Qaeda operatives still remained in Afghanistan and Pakistan, while others had returned to their countries of origin to plan new attacks.